graph instead of a tree

How might the protocol enable hats to take a more generalized graph structure rather than specifically a tree structure?

Currently, hats can only be created in the form of a tree structure. This is efficient, and it covers many organizational structures (and yet more can work with it via hatter contracts), yet it imposes some constraints where perhaps there may be circumstances or use cases where fewer constraints may be valuable.

thought starters:
- create a HatAdmin struct that can take several adjustable properties to define the type of edge between the admin the hat
- move to a more explicit edge and node approach